Polymers such as polyethylene or polypropylene are generally produced in industrial quantities by petrochemical facilities, which have ready access to the monomeric feedstocks. The polymerization process itself is usually exothermic, and is typically performed in closed systems where temperature and pressure can be regulated to maximize production. As with any such closed system where heat is generated, it is necessary to remove most of the heat in order to control the polymerization temperature. Thus the reactors in which the polymerisation is conducted are usually provided with coolant systems to remove the heat.
A typical form of coolant system comprises a jacket surrounding the polymerisation reactor through which is passed coolant to remove the heat from the reactor. The coolant itself usually circulates via a heat exchanger, where the heat removed from the reactor is given up.
Although many polymerisation processes involve the use of only a single polymerisation reactor, increasing numbers utilise two or more reactors, the products of which are combined. The reactors may produce identical or different products, and may be arranged in parallel or in series. A common application is to make one polymer in a first reactor, and then transfer the polymer to a second reactor where a further polymer is made in the presence of the first polymer (so-called “multimodal” polymerisation). Such reactor systems may comprise liquid-phase or gas-phase reactors or a combination thereof.
Many multiple reactor systems employ one or more loop reactors, which are of a continuous tubular construction comprising at least two, for example four, vertical sections and at least two, for example four, horizontal sections. In such a reactor, reactants are fed into the continuous tube containing a solvent and a catalyst. The admixture of diluent, reactants, and catalyst are continuously recycled around the tube, with the reaction product being continuously removed. The heat of polymerisation is typically removed using indirect exchange with a cooling medium, preferably water, in jackets surrounding at least part of the tubular loop reactor. A typical loop reactor is shown in FIG. 8 of WO 2006/026493, and a conventional system for cooling such a reactor is shown in FIG. 9 of the same patent application. This shows a cooling circuit comprising a cold coolant supply to the reactor jackets, and a return supply of warmer coolant from the jackets, “carrying” the heat from the reactor. The heat is removed from the coolant in heat exchangers, and the re-cooled coolant recirculates into the reactor jackets. The coolant is cooled in heat exchangers and recirculated rather than being supplied directly from a cold source such as a cooling tower so as to avoid fouling caused by untreated or only partially treated water: the circulating coolant (usually water) in the circuit can be treated.
It is usual for the reactors in multiple reactor polymerisations each to have its own independent cooling circuit and heat exchanger, since the cooling requirements of each reactor, where different polymerisation reactions are taking place, are usually different. However in U.S. Pat. No. 6,235,852 a single heat exchanger is used to supply the cooling circuits for two or more reactors. This is said to be possible because of the lower heat removal requirements in subsequent reactors due to ageing of the catalyst. A bypass line directly from the cooling circuit of one reactor to the other is also disclosed. We have found that it is possible to improve the above design by integrating the two cooling circuits further so that they are effectively a single circuit, thereby providing a simpler and more controllable cooling system.